1. Field of the Invention
The present invention relates to solder alloys which are completely free from lead and are suitable for use in soldering of electronic devices without producing thermal damage.
2. Description of the Related Art
Snxe2x80x94Pb alloys have long been used for soldering in the electronics industry, and they are still the most popular alloys for soldering electronic devices to printed circuit boards or other substrates.
When electronic appliances such as televisions, radios, audio or video recorders, computers, pocket telephones, and copying or printing machines are to be discarded, they are typically disposed of in landfills, since they commonly include large amounts of synthetic resins (used for housings and printed circuit boards) and metals (used for frames and connecting wires) which are not suitable for incineration.
In recent years, acid rain (the phenomenon in which rain becomes highly acidic due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as coals, gasolines, and fuel (heavy) oils) has become increasingly serious. Acid rain causes the solders used in discarded electronic appliances present in landfills to dissolve and contaminate groundwater. If groundwater contaminated with lead is ingested by a person for many years, the accumulation of lead in the person""s body may result in lead poisoning. For this reason, there is a need for a lead-free solder alloy in the electronics industry.
Conventional lead-free solder alloys are Sn-based alloys such as Snxe2x80x94Ag and Snxe2x80x94Sb alloys. Of Snxe2x80x94Ag alloys, an Sn3.5-Ag alloy has a eutectic composition with a melting temperature of 221xc2x0 C. Even if this composition, which has the lowest melting temperature among Snxe2x80x94Ag alloys, is used as a solder alloy, the soldering temperature will be as high as from 260xc2x0 C. to 280xc2x0 C., which may cause thermal damage to heat-sensitive electronic devices during soldering, thereby deteriorating their functions or rupturing the devices. Of Snxe2x80x94Sb alloys, an Sn5-Sb alloy has the lowest melting temperature, but its melting temperature is still as high as 235xc2x0 C. at the solidus line and 240xc2x0 C. at the liquidus line. Therefore, the soldering temperature is in the range of from 280xc2x0 C. to 300xc2x0 C., which is higher than that of an Sn3.5-Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided.
In view of the relatively high melting temperatures of Snxe2x80x94Ag and Snxe2x80x94Sb alloys, many attempts to lower their melting temperatures have been proposed. See, for example, Japanese Patent Applications Laid-Open (JP A1) Nos. 6-15476(1994), 6-344180(1994), 7-1178(1995), 7-40079(1995), and 7-51883(1995).
The solder alloys disclosed in these Japanese patent applications contain a large proportion of Bi and/or In (indium) in order to lower their melting temperatures. Although Bi and In are both effective for decreasing the melting temperatures of Snxe2x80x94Ag and Snxe2x80x94Sb solder alloys, the addition of Bi and/or In in a large amount is accompanied by a number of problems. Addition of Bi in a large proportion makes the solder alloys very hard and brittle. As a result, it is impossible or difficult to subject the solder alloys to plastic working to form wire, and when the solder alloys are used to solder electronic devices, the soldered joints may be readily detached when subjected to only a slight impact. Addition of indium in a large proportion to solder alloys is undesirable due to its very high cost.
In order to avoid thermal damage to electronic devices during soldering, the soldering temperature should generally be at most 250xc2x0 C. In order to perform soldering at a temperature of at most 250xc2x0 C., it is desirable that the liquidus temperature of the solder alloy be at most 220xc2x0 C. and preferably at most 200xc2x0 C.
However, when attempting to lower the melting temperatures of Snxe2x80x94Ag and Snxe2x80x94Sb solder alloys by addition of Bi and/or In, it is difficult to decrease the liquidus temperature of the alloys to 200xc2x0 C. or below unless Bi and/or In is added in a large amount. Furthermore, even though it is possible to provide a solder alloy having a liquidus temperature lowered to 200xc2x0 C. or less by addition of Bi and/or In, the solidus temperature thereof, at which solidification of the alloy is completed, may be excessively lowered, so that it takes a prolonged period of time to completely solidify the solder alloy in soldered joints formed by soldering. As a result, if the soldered joints are subjected to any vibration or impact before they are completely solidified, they may crack.
Another problem of conventional lead-free solder alloys is that those lead-free alloys having liquidus temperatures which are low enough to be close to their solidus temperatures do not have satisfactory mechanical properties such as tensile strength and elongation, thereby forming soldered joints which have poor bonding strength or which are liable to be detached upon impact.
It is an object of the present invention to provide lead-free solder alloys having a liquidus temperature which is less than 220xc2x0 C. and preferably less than 200xc2x0 C. and a solidus temperature, at which solidification of the alloy is completed or substantially completed, which is 160xc2x0 C. or higher, and preferably 170xc2x0 C. or higher.
It is another object of the present invention to provide lead-free solder alloys which have good bonding strength when used for soldering.
A more specific object of the present invention is to provide lead-free solder alloys having the following properties.
1) The alloys can be used at a soldering temperature below 250xc2x0 C. and preferably from 230xc2x0 C. to 240xc2x0 C. so as to prevent thermal damage to heat-sensitive electronic devices during soldering.
2) The alloys have excellent solderability.
3) The alloys have a narrow solidification temperature range between the liquidus and solidus temperatures such that the alloy""s are rapidly solidified after soldering in order to prevent the resulting soldered joints from cracking when subjected to vibration or an impact immediately after soldering, the temperature range being close to the eutectic temperature of an Snxe2x80x94Pb alloy (183xc2x0 C.).
4) The alloys produce soldered joints having a bonding strength which is high enough to prevent the joints from being detached when subjected to an impact.
5) The alloys can be easily subjected to plastic working to form wire so that the alloys can be used for soldering with a soldering iron.
The present inventors found that alloys consisting essentially of Zn, Bi, optionally one of P and Ge, and a balance of Sn in specific proportions can provide solder alloys having a low liquidus temperature which enables dip soldering to be performed in a temperature range at which electronic components being soldered will not undergo thermal damage. Furthermore, it was found that by appropriately adjusting the proportions of Zn and Bi, the solidus temperatures of the alloys can be close to their liquidus temperatures, thereby enabling molten solder to rapidly solidify following soldering to avoid problems such as cracking or detachment of soldered joints. In addition, the alloys have a tensile strength and ductility which enables them to be plastically formed into wire suitable for use with a soldering iron. Thus, these solder alloys can be satisfactorily used in place of conventional Snxe2x80x94Pb alloys, and because these solder alloys are lead free, they prevent contamination of groundwater by lead which occurs with conventional Snxe2x80x94Pb alloys.
According to one aspect of the present invention, a lead-free solder alloy consists essentially of from 5 to 9 mass % of Zn, from 2 to 15 mass % of Bi, and a balance of Sn.
According to another aspect of the present invention, a lead-free solder alloy consists essentially of from 5 to 9 mass % of Zn, from 2 to 15 mass % of Bi, P and/or Ge, and a balance of Sn.